1. Field of the Invention
This invention concerns a technique and an apparatus that is used for the measurement of bi-axial strain in printed paperboard or paperboard based laminates. Such strain information is useful for determining the margin to failure of various features of laminated paperboard cartons, such as those used in the liquid packaging industry. Knowing such margins of failure is crucial for ensuring integrity of the microbiological barrier in aseptic liquid packaging.
2. Description of the Related Art
It is known that visual access to the web and carton forming area is hindered by packaging machinery, making in-process strain measurement very difficult. Furthermore, the geometry of the web changes considerably during the forming process, thereby making traditional optical strain measurement techniques unworkable.
It is also known in the current practice of strain measurement to use a digital camera to photograph an area having test points or reference points marked. Exemplary of such prior art is U.S. Pat. No. 5,726,907 (""907) to D. L. Davidson et al., entitled xe2x80x9cBi-Axial Non-Contacting Strain Measurement Using Machine Visionxe2x80x9d. As discussed above, the ""907 reference uses a digital camera to photograph an area of a sample having test points or reference points marked, which allow any subsequent strain of the sample to be determined by relative movement of the test points from one another. Using a digital computer, the measurement of strain within the sample may be automated. While the above principle works well for unidirectional strains, it will only work for bi-axial strains if the geometry between the camera and sample is preserved throughout the strain process. If the focal length and/or incident angle of the focal axis changes in an uncertain manner, little useful information can be deduced.
Finally, it is known to photograph a specimen in unstrained states. Exemplary of such prior art are U.S. Pat. No. 5,757,473 (""473) to H. Kanduth et al., entitled xe2x80x9cOptical Strain Sensor for the Measurement of Microdeformations of Surfacesxe2x80x9d, U.S. Pat. No. 5,812,251 (""251) to A. Manesh, entitled xe2x80x9cElectro-Optic Strain Gages and Transducerxe2x80x9d, and U.S. Pat. No. 5,920,383 (""383) to F. Chen et al., entitled xe2x80x9cMicroscopic Digital Imagining Strain Gauge.xe2x80x9d Again, while the ""473, ""251, and ""383 references employ a photograph of the specimen in strained and unstrained conditions, the access to the specimen surface must be unhindered to allow access to the microscope lenses, prisms, surface attachments, etc.
It is apparent from the above that there exists a need in the art for a measurement apparatus which can measure paperboard strain in a confined area, but which at the same time has a very small footprint and can make strain measurements on a surface with a small radius of curvature. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.
Generally speaking, this invention fulfills these needs by providing an apparatus for measuring paperboard strain, comprising a fiber optic scope means, a fiber optic tube means operatively connected to the fiber optic scope means for retaining the fiber optic scope means, and an adjustable stand-off means operatively connected to the tube means for constraining a focal length between an end of the fiber optic scope means and a surface of the paperboard.
In certain preferred embodiments, the fiber optic scope means includes a fixed, elongated fiber optic. Also, the tube means includes a bracket means. Finally, the adjustable standoff means includes an adjustment means.
In another preferred embodiment, a fiber optic scope fitted with an adjustable standoff means provides an ideal tool for the measurement of strain in the sample which is subjected to an undetermined stress field. This device is capable of maintaining a fixed geometry at a fixed focal length from the surface of the paperboard sample.
The preferred apparatus, according to this invention, offers the following advantages: lightness in weight; ease of assembly and repair; excellent strain measurement characteristics; excellent durability; excellent alignment of the fiber optic; portability; and good economy. In fact, in many of the preferred embodiments, these factors of lightness in weight, ease of assembly, excellent strain measurement, durability, fiber optic alignment, and portability are optimized to the extent that is considerably higher than heretofore achieved in prior, known apparatus for measuring paperboard strain.
The above and other features of the present invention, which will become more apparent as the description proceeds, are best understood by considering the following detailed description, in conjunction with the accompanying drawings, wherein like characters represent like parts throughout the several views and in which: